Photovoltaic cells, which receive light and convert the light into electric energy, are made by depositing various layers of materials on a substrate. The most common substrate material used in the manufacture of thin film Cu(In, Ga)Se (CIGS) photovoltaic cells is glass because glass provides a good balance of properties at moderate cost. In particular, glass provides good mechanical support; is thermally and chemically stable to the processes used to deposit various layers of the thin film photovoltaic cell onto the substrate; is electrically insulating; and provides excellent barrier properties to protect the water and oxygen-sensitive layers of the photovoltaic cell. In addition, glass has a smooth surface, which enables the surface of a film that is placed on the glass to be made relatively smooth in turn.
Glass substrates also have some disadvantages. They are heavy, prone to breakage, and generally too rigid to be used in potentially more economical roll-to-roll processes. These disadvantages have motivated the search for alternative substrates. Metal foils can be used as substrates, but have the distinct disadvantage that they are electrically conductive and are also heavy. Organic polymers, such as polyimides, are amenable to use in roll-to-roll processes and can be weight-saving substrates in many applications, but they do not have sufficient thermal and dimensional stability at the high temperatures, for example above 500° C., which are needed to realize higher photovoltaic efficiencies for CIGS based devices. The use of mica as a substrate has also been reported.
U.S. Pat. No. 7,663,056 discloses a chalcopyrite type solar cell having a mica aggregate substrate formed by binding mica particulates with a resin.
US Published Patent Application No. 2009/0133749 discloses a chalcopyrite solar cell having a mica substrate or a laminated mica substrate.
US Published Patent Application No. 2009/0202806 discloses an inorganic layered compound film comprising an oriented denatured clay and exhibiting high water resistance, excellent pliability, excellent gas barrier properties and high heat resistance. The clay used in the denatured clay may be mica. The denatured clay may be reacted with a silylating agent.
US Published Patent Application No. 2009/0205715 discloses a solar cell which includes a substrate of mica or material containing mica; an intermediate layer for smoothing or planarizing a surface of the substrate, which is formed on the substrate; a binder layer formed on the intermediate layer; a metallic lower part electrode layer formed on the binder layer; a p-type light absorbing layer formed on the metallic lower part electrode layer, and made of chalcopyrite based material; an n-type buffer layer formed on the light absorbing layer; and an n-type transparent electrode layer formed on the buffer layer.
Mica substrates that are flexible, tolerant of the high temperatures used to create the photoactive layers, inexpensive, and suitable for use in roll-to-roll processes continue to be sought. Mica substrates which furthermore have improved or suitable tear initiation resistance, improved water resistance, and a thermal expansion coefficient similar to that of the metal used in a back electrode of a photovoltaic cell are especially sought.